A touch screen is a device that can detect an object in contact with or in proximity to a display area. The display area can be covered with a touch-sensitive matrix that can detect a user's touch by way of a finger or stylus, for example. Touch screens are used in various applications such as mobile phones and other mobile devices. A touch screen may enable various types of user input, such as touch selection of items on the screen or alphanumeric input via a displayed virtual keypad. Touch screens can measure various parameters of the user's touch, such as the location, duration, etc.
One type of touch screen is a capacitive touch screen. A capacitive touch screen may include a matrix of conductive rows and conductive columns overlaid on the display area. The conductive rows and the conductive columns are spaced apart from one another so as to not make contact. The capacitive touch screen may be used for mutual capacitance sensing.
In mutual capacitance sensing, the capacitance between each row and column of the matrix may be sensed. A change in the sensed capacitance between a row and a column may indicate that an object, such as a finger, is touching the screen or is in proximity to the screen near the region of intersection of the row and column. Mutual capacitance sensing circuits employ a “forcing” signal applied to a column conductor of the capacitive touch matrix and sensing of the coupled signal on respective row conductors (or vice-versa).
To perform noise sensing in mutual capacitance sensing, so as to be able to determine a best available frequency for the mutual capacitance sensing, the row conductors are sensed without the application of the force signal to the column conductors, at multiple frequencies. The frequency with the lowest noise level is then used for the next mutual capacitance sensing during which the forcing signal is applied to the column conductor.
While this noise sensing technique does work, noise data is not available every data frame, and the noise itself may change significantly from frame to frame. Therefore, improvement in the area of noise sensing in mutual capacitance sensing is needed.